It is known that emerging sugar beet plants are often affected by parasitic fungi present in the soil, such as Pythium ultimum, Pythium debaryanum, various species of the Aphanomyces and Fusarium genus, as well as by Rhizoctonia solani and others. Pythium oligandrum had been considered a mild parasite too. Said parasites are the cause of the known disease called usually "damping off" or "black leg", capable of destroying a great deal of emerging sugar beet plants, particularly in a cold weather. The disease attacks mainly the sprouting up seeds, mostly prior to the sprout penetrating the soil surface. The disease usually dies away as soon as the first pair of true leaves is formed. Thereby it is possible to explain the fact that the damages caused by the damping off were not too serious in the past when natural seed clusters were seeded in an amount of 25 to 30 kg per hektar. Then about 1,500,000 of plants per hektar emerged and superfluous seedlings were removed simultaneously with weak or apparently diseased plants at a stage when true leaves were already grown and the disease passed away. The method was, however, uneconomical, demanding a great deal of manual labor. Nowadays considerably lower doses of ground-off or genetically one-bud seeds are used so that only about 200,000 plants per hektar emerge and no superfluous plants need to be removed. The method is, as such, very economical, the damping-off, however, appears to be much more damaging than in the past, often decreasing the crops quite seriously. Therefore, the damping-off of emerging sugar beet becomes the limiting factor barring a more extensive use of modern cultivating methods, at which the sugar beet is seeded in definitive distances and requires no manual labor. The only way to protect emerging sugar beet plants against "damping off" was, until now, soaking or dressing of the seed with fungicides containing mostly organic derivatives of mercury and sulfur.
Said fungicides are effective but simultaneously noxious for animals and for human health, and it can be expected that their use will be successively reduced to protect the biosphere from a pollution.
Mercury accumulates in the soil and penetrates into plants, herbivorous animals and fish in water-courses and gets, through the nutrition chain, into human body where it causes long lasting degenerative damages. Sulfur compounds, though often considered comparatively harmless, are nevertheless toxic enough. So e.g. tetramethylthiuram disulphide (TMTD) has a rather low LD.sub.50 -865 mg per kg of live weight. The dust of pulverulent TMTD, being about 5 times more toxic than some systemic fungicides, endangers particularly the labor at seed dressers and seeding machines, affecting seriously human nervous system. There are known no effective and simultaneously fully harmless fungicides capable of protecting sugar beet against the "black leg" disease.
Said disease is caused mainly by micromycetes of the Pythium genus, particularly by Pythium ultimum and P. debaryanum.
Pythium oligandrum was discovered in 1930 on the pea roots as one of pathogenic agents. The discoverer, C. Drechsler, has found, when studying fungi causing rot of roots and other parts of products and fruits of various cultivated plants such as tomatoes and watermelons in the South of the U.S.A. already in 1943 that Pythium oligandrum parasites on various other pathogenic species of the same genus such as on Pythium ultimum and Pythium debaryanum (Drechsler, C., Phytopathology 33, 1943, 4, 261-299). More recently J. W. Deacon studied analogical biotic relations of Pythium oligandrum to microorganisms parasiting on grain (e.g. Gaeumannomyces graminis) and mentioned also that he succeeded in controlling a wheat disease caused by Pythium ultimum by inoculating the soil simultaneously with Pythium oligandrum and Pythium ultimum (Trans. Br. Soc. 66 /3/, 383-391). Said author offered his opinion that the result might be caused by nutrition competition or, perhaps, also by direct weakening of the parasitic Pythium ultimum by the mycoparasitic Pythium oligandrum. The existence of mycoparasitism in the rhisosphere of sugar beet was neither proved nor disclosed as yet.
The endeavour to check the damping-off was directed rather to the breeding of resistant sugar beet strains, compare e.g. F. Koch (Gruppo gior. edagric. 20, 1974, 1-2, 8-12), which author included, in agreement with others, Pythium oligandrum into the same group of pathogens as e.g. Pythium ultimum and Pythium irregulare. Another repeatedly tried way to control said disease was the utilization of the antibiosis. There were produced and tested several preparations based on the production of antiobiotics by certain micromycetes such as Trichoderma viride. There are, however, serious objections from the medical point of view against the use of antibiotics in agriculture especially that in repeated contact of agricultural workers with antibiotics in lower-effective doses, resistant strains of pathogenic microorganisms can be developed. Moreover, the suggested antibiotics possess but a narrow spectrum of activity and their effect in the practice is rather uncertain, depending on the weather and soil condition, e.g. on the humidity and on the contingent presence of antagonists in the soil. Inoculated microorganisms capable of producing antibiotics succumb not only to antagonists but also to parasites and are even liable to stimulate the growth of some pathogens.